1. Field of the Invention
This invention relates to a pneumatic radial tire for a construction vehicle such as a large-size dump truck or the like running on rough roads such as large-scale civil engineering sites, stone pits and the like and subjected to severe working conditions under a heavy load. More particularly it relates to an improvement of bead portion durability in such a tire.
2. Description of the Related Art
The pneumatic radial tire for a construction vehicle is used at such a state that the range of speed is low, the load is high (under the same internal pressure) and the internal pressure is low (under the same load) as compared with pneumatic radial tire for truck and bus of the same size.
In the pneumatic radial tire for a construction vehicle, however, a bending quantity ranging from a sidewall portion to a bead portion is large as compared with other heavy duty tires, and further a large change of load is caused during the running on roads having considerably uneven surfaces, so that problems at the bead portion are frequently caused. Therefore, it is necessary to improve the durability of the bead portion in order to enhance the reliability of the pneumatic radial tire for construction vehicle.
In general, the conventional tire of this type comprises as a body reinforcement a carcass of a radial structure comprised of at least one layer of rubberized steel cords toroidally extending between a pair of bead cores, which is wound around each bead core from inside toward outside to form a turnup portion. Further, as shown in FIG. 5, at least two chafers (reinforcing layers) each comprised of rubberized steel cord fabric or nylon cord fabric and having different cord angles with each other, or a combination of these fabrics are arranged in a bead portion including the bead core and the carcass turnup portion surrounding therearound as a skeleton structure along the carcass turnup portion to form a reinforcement for the bead portion.
Although the rigidity of the bead portion is improved by the reinforcement using the above chafers, when the tire is repeatedly subjected to compression deformation during running of the tire, a large compression strain is applied from an outer end of a rim flange to an outward portion of the tire in the radial direction, which is conspicuous in the tire of this type, whereby fatigue breakage is frequently and easily caused in the carcass turnup portion and the chafer portion. Furthermore, the rigidity required in the bead portion can not sufficiently be obtained, so that there is a risk of separating rubber from the carcass turnup portion made from steel cords and the end of the chafer to cause separation failure.
The bending rigidity can be improved and also the compression deformation can somewhat be mitigated by increasing the thickness of the stiffener rubber located between the carcass ply and its turnup portion above the bead core. When the compression deformation is repeatedly suffered as previously mentioned, the breakage is caused at the carcass turnup portion or the separation failure is caused in the vicinity of the carcass turnup portion, and hence the temperature rise at the bead portion is not particularly avoided, which promotes the separation failure and consequently the above effect is off-set and the durability of the bead portion can not be improved.